Nonwoven webs typically are formed using a meltblowing process in which filaments are extruded from a series of small orifices while being attenuated into fibers using hot air or other attenuating fluid. The attenuated fibers are formed into a web on a remotely-located collector or other suitable surface.
There has been an ongoing effort to improve the uniformity of nonwoven webs. Web uniformity typically is evaluated based on factors such as basis weight, average fiber diameter, web thickness or porosity. Process variables such as material throughput, air flow rate, die to collector distance, and the like can be altered or controlled to improve nonwoven web uniformity. In addition, changes can be made in the design of the meltblowing apparatus. References describing such measures include U.S. Pat. Nos. 4,889,476, 5,236,641, 5,248,247, 5,260,003, 5,582,907, 5,728,407, 5,891,482 and 5,993,943.
The attenuating fluid typically is supplied to a manifold (e.g., an air manifold) attached to the side of the die body, optionally sent through a tortuous path in the manifold or in the die body, and then sent through attenuating fluid flow channels to exit near the filament orifices so that the attenuating fluid can impinge upon and draw down the extruded filaments into fibers. Representative manifolds, tortuous paths and flow channels are shown in, for example, U.S. Pat. Nos. 4,889,476, 5,080,569, 5,098,636, 5,248,247, 5,260,003, 5,580,581, 5,607,701, 5,632,938, 5,667,749, 5,711,970, 5,725,812, 6,001,303 and 6,182,732.
Despite many years of effort by various researchers, fabrication of commercially suitable nonwoven webs still requires careful adjustment of the process variables and meltblowing apparatus parameters, and frequently requires that trial and error runs be performed in order to obtain satisfactory results. Fabrication of wide melt blown nonwoven webs with uniform properties can be especially difficult.